1. Field of the Invention
This invention relates to a toner for forming toner images in image forming processes such as electrophotography, electrostatic printing, magnetic recording and toner jet recording, and an image forming method employing such a toner. More particularly, this invention relates to a toner for developing electrostatic images which is used in a fixing system in which visible images formed out of toner are heat-fixed to recording mediums, and an image forming method employing such a toner.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are conventionally known as electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a recording medium such as paper by an direct or indirect means as the occasion demands, followed by fixing by the action of heat, pressure or solvent vapor. The toner that has not transferred thereto and has remained on the photosensitive member is removed by cleaning by various means, and then the above process is repeated.
A usual full-color image forming method will be described. A photosensitive member (electrostatic latent image bearing member) such as a photosensitive drum is electrostatically uniformly charged by means of a primary charging assembly, and imagewise exposure is carried out using laser light modulated by magenta image signals of an original, to form an electrostatic latent image on the photosensitive drum. The electrostatic latent image is developed by means of a magenta developing assembly holding a magenta toner, to form a magenta toner image. Next, to a recording medium transported, the magenta toner image developed on the photosensitive drum is transferred by a direct or indirect means by means of a transfer charging assembly.
The photosensitive drum on which the electrostatic latent image has been developed is decharged by a residual charge eliminator, and is further cleaned through a cleaning means. Thereafter, it is again electrostatically charged by the primary charging assembly, and a cyan toner image is similarly formed. The cyan toner image is transferred to the recording medium on which the magenta toner image has been transferred, and then a yellow toner image and a black toner image are successively formed and developed so that the four color toner images are transferred to the recording medium. The recording medium having these four color toner images is passed through a fixing roller so that they are fixed to the recording medium by the action of heat and pressure. Thus, a full-color image is formed.
In recent years, such image forming apparatus are not only used as copying machines for office work to merely take copies of originals, but also have began to be used in the field of laser beam printers (LBPs) serving as the output of computers and in the field of personal copying (PC) of private use.
In addition to the field as typified by LBPs and PC, such apparatus are also being rapidly expanded to plain-paper facsimile machines to which basic engines are applied.
Under such circumstances, the apparatus are more severely sought to be made small-sized, light-weight, high-speed, image high-quality and highly reliable, and such machines have now been composed of more simple components in various respects. As the result, a higher performance has become required for toners, and superior machines can now no longer be accomplished unless improvement in the performance of toners is achieved. In recent years, with a need for various modes of copying, demand for color copying is rapidly increasing. In order to more faithfully copy original color images, it is sought to achieve a much higher image quality and a much higher resolution. Moreover, there is an increasing demand for the copying of double-side color originals.
From these viewpoints, as the toners used in the color image forming process, it is preferable to use toners having good melt properties and color-mixing properties when heat is applied thereto and also having a low softening point and high sharp-melt properties in a low melt viscosity.
Use of such sharp-melt toners makes it possible to broaden the range of color reproduction of copied matter and obtain color copies faithful to original images.
Color toners having such high sharp-melt properties, however, is so high in affinity for the fixing roller that it tends to cause offset with respect to the fixing roller at the time of fixing.
In particular, in the case of a fixing assembly in full-color image forming apparatus, an increase in toner layer thickness tends to cause the offset since a plurality of toner layers corresponding to magenta, cyan, yellow and black are formed on the recording medium.
In order to allow no toner to adhere to the surface of the fixing roller, a measure has been conventionally taken in which the roller surface is formed out of a material, such as silicon rubber or a fluorine resin, having an excellent releasability to toner, and, in order to prevent offset and to prevent fatigue of the roller surface, its surface is further covered with a thin film formed using a fluid having a high releasability as exemplified by silicone oil or fluorine oil. However, although this method is very effective in view of the prevention of the offset of toner, it requires a device for feeding an anti-offset fluid, and hence has such a problem that a complicated fixing assembly is required. In addition, the application of oil may bring about separation of layers on the fixing roller, and consequently, shorten the lifetime of the fixing roller.
Accordingly, based on the idea that the fluid for preventing offset should be fed from the inside of toner particles at the time of heat fixing, without use of any device for feeding silicone oil, a method has been proposed in which a release agent such as a low-molecular-weight polyethylene or a low-molecular-weight polypropylene is added in toner particles.
Japanese Patent Publications No. 52-3304 and No. 3305 and Japanese Patent Publication 57-52574 disclose that as the release agent a wax is incorporated into toner particles.
Japanese Patent Applications Laid-open No. 3-50559, No. 2-79860, No. 1-109359, No. 62-14166, No. 61-273554, No. 61-94062, No. 61-138259, No. 60-252361, No. 60-252360 and No. 60-217366 disclose techniques for incorporating waxes.
In the case of black toners, release agents having a relatively high crystallizability as typified by polyethylene wax and polypropylene wax can be used in order to improve high-temperature anti-offset properties at the time of fixing. However, in the case of full-color toners, this crystallizability of release agents may cause great damage to the transparency of OHP (overhead projector) toner images when outputted. Moreover, the wax may cause a lowering of blocking resistance of toners, and a lowering of developing performance because of migration of wax toward toner particle surfaces when toners are exposed to heat as a result of temperature rise in image forming apparatus such as printers and copying machines and also when toners are left standing for a long term.
To cope with such problems, various improvements are attempted from the aspect of binder resin. More specifically, a cross-linking component or a high-molecular-weight component is used in a binder resin in a larger quantity so that the high-temperature anti-offset properties at the time of fixing can be improved.
This method can certainly improve high-temperature anti-offset properties to a certain extent and also can be effective for improving durability such that external additives are prevented from being buried in toner particle surfaces and toners are prevented from melt-adhereing to the photosensitive member and toner carrying member.
However, this method conflicts with the improvement of grindability and low-temperature fixing performance of toners, and there is still room for improvement in order to achieve both the high-temperature anti-offset properties or durability and the low-temperature fixing performance.
Accordingly, to solve the above problems, much hope has been put in the development of novel toners.
To cope with the above subject, a toner produced by suspension polymerization is proposed (Japanese Patent Publication No. 36-10231). In this suspension polymerization, polymerizable monomers and a colorant (and also optionally a polymerization initiator, a cross-linking agent, a charge control agent and other additives) are uniformly dissolved or dispersed to form a monomer composition, and thereafter this monomer composition is dispersed in a continuous phase (e.g., an aqueous phase) containing a dispersion stabilizer, by means of a suitable stirrer to simultaneously carry out polymerization reaction to obtain toner particles having the desired particle diameters.
In this suspension polymerization, droplets of the monomer composition are produced in a dispersion medium having a large polarity such as water, and hence what is called core/shell structure can be formed in which components having polar groups, contained in the monomer composition, tend to present at the surface layer portions which are interfaces with the aqueous phase and non-polar components are not present at the surface layer portions.
Because of encapsulation of the release agent wax component, the toner produced by polymerization makes it possible to achieve both the low-temperature fixing performance or blocking resistance and the high-temperature anti-offset properties and also makes it possible to prevent high-temperature offset without applying any oil release agent to the fixing roller.
Toners for developing electrostatic images commonly contain a binder resin and a colorant as essential components, and various methods for improving binder resins are proposed for the purpose of improving the developing performance, fixing performance, storage stability and environmental stability of toners. For example, with regard to the above toners produced polymerization, a method is presented in which shells of a resin having a relatively low glass transition temperature (Tg) are covered with a resin having a relatively high Tg in order to achieve both the low-temperature fixing performance and the storage stability (e.g., Japanese Patent Application Laid-open No. 5-197203). However, most resins having a relatively high Tg which are used therein are polar resins having a moisture absorption, such as polyesters. Even though such resins can achieve both the low-temperature fixing performance and the storage stability, they have often caused a problem on charging stability resistant to environment variations.
Moreover, toners are commonly known to undergo deterioration caused by external additives that may be buried in toner particle surfaces when images are printed on many sheets, to adversely affect the images. As a means for improving the running performance of toners, a method is available in which the binder resin is made to have a higher mechanical strength. Since, however, problems may actually arise on the grindability of the binder resin and the fixing performance of toners, it is commonly difficult to use such a tough resin as a binder resin.
As resins having superior mechanical strength, electrical characteristics and aging resistance (weatherability), polycarbonates are commonly widely known and are used in various purposes. Some methods in which polycarbonates are used as binder resins are disclosed also in respect of toners.
For example, Japanese Patent Application Laid-open No. 46-28588 discloses an image forming method making use of a specific polycarbonate copolymer and a granular carrier. According to this publication, a toner having a superior blocking resistance can be obtained by using a specific polycarbonate copolymer as the binder resin. However, according to this publication, a polycarbonate copolymer having a glass transition temperature of from 70 to 95xc2x0 C. is used as the binder resin and also any wax component is not contained in the toner, resulting in a very poor low-temperature fixing performance. Thus, there is room for improvement. The publication also has no description as to any influence on electrophotographic performance that may be caused by impurities contained in the polycarbonate copolymer. The publication still also discloses, in Examples, processes for producing toners by spray drying and pulverization, but has no disclosure at all as to differences in transfer performance of toner images from the electrostatic latent image bearing member to the recording medium and differences in charging uniformity, which are ascribable to the shapes of the toners obtained.
Japanese Patent Application Laid-open No. 63-208863 discloses a method in which a polycarbonate terpolymer with a specific structure, having a glass transition temperature of about 50xc2x0 C., is used as a binder resin of a toner for flash fixing. According to this publication, the toner can be free from any bad smell and eluted matter because the binder resin polycarbonate terpolymer does not thermally decompose during flash fixing, and a toner having a good fixing performance can be obtained even though it contains no wax component. On the other hand, however, since only the polycarbonate terpolymer having a low glass transition temperature is used as the binder resin, the toner has not reached satisfactory levels in respect of blocking resistance and running performance. Also, since the toner is one designed for flash fixing, it is difficult for the toner to be applied to a type of fixing assembly, e.g., in which the toner comes into contact with a heating element as in heat-roll fixing.
U.S. Pat. No. 4,457,998 also discloses a toner having a structure wherein a linear binder resin is incorporated in a binder resin cross-linked in a high degree, and states that a polycarbonate copolymer can be used as the highly cross-linked resin or the linear binder resin or as both of the two. In the specification of this publication, however, there is no disclosure of an example where the polycarbonate copolymer is used, and it is unclear about any effect obtainable when the polycarbonate copolymer is used as the binder resin.
Japanese Patent Application Laid-Open No. 5-273782 discloses that filming can be prevented by using a toner with a value of Izot impact strength of 2 to 500 kgxc2x7cm/cm when made into a plate in an image forming method using a developing roller in which many minute closed electric fields are formed near the surface of the developing roller. It is said that a mixture of styrene-acrylic resin and polycarbonate may be used as a binder resin for the toner. However, in this publication, there is no description about polycarbonate. In addition, it has conducted no investigation of a component which has a repeating unit of polycarbonate and is contained in components having a molecular weight of 1,000 or less in a molecular weight distribution as measured by GPC, and the molecular weight of the polycarbonate.
Japanese Patent Application Laid-open No. 6-43688 discloses a method in which a polycarbonate copolymer having a specific structure that exhibits thermotropic liquid-crystal properties is used as a binder resin. The polycarbonate copolymer that exhibits thermotropic liquid-crystal properties usually has a high crystallizability, shows a gentle heat softening behavior up to its melting point, and further abruptly liquefies (melts) upon temperature rise to cause a decrease in viscosity and a drop in temperature. Because of such properties, the toner in which such a polycarbonate copolymer is used as the binder resin, even though it contains no wax component, can be fixed at a low energy while maintaining the grindability and blocking resistance. However, since the toner disclosed in this publication is constituted only of one kind of binder resin, the toner is so low in a viscosity at the time of its melting that what is called high-temperature offset is brought about, where the molten toner adheres to fixing members such as heat rolls. Such a problem remains unsettled. Moreover, the publication has no specific description as to any influence on electrophotographic performance that may be caused by impurities contained in the polycarbonate copolymer and as to the shape of toner particles.
As previously mentioned, in recent years, among users there is an increasing demand for the copying of double-side originals or the double-side copying of single-side orginals. Thus, double-side images having a higher image quality and a higher reliability are required for such purpose.
Among various problems of conventional techniques for double-side color copying, one of the most important subjects is paper curl that occurs after the fixing on one side. If this paper curl greatly occurs, the fixed images may have too poor transport performance to obtain images having a high image quality and a high reliability. To cope with this, toners are required to have, e.g., the performance of providing high-quality images satisfying image density, color reproducibility and so forth are obtainable in such a state that the toner is transferred to the recording medium in a small quantity. For this end, it becomes necessary to improve the coloring power of the toners themselves. In the double-side copying, since images that pass through a fixing assembly twice occur, it is required to be more improved in the high-temperature anti-offset properties.
In conventional full-color copying machines, commonly used are a method in which four photosensitive members and a belt-like transfer member are used, where electrostatic images formed on the photosensitive members are developed by the use of cyan, magenta, yellow and black toners and thereafter a recording medium is transported between the photosensitive members and the belt-like transfer member to transfer toner images by straight-pass, forming a full-color image, and a method in which a recording medium is wound around the surface of a transfer member by electrostatic force or by a mechanical means such as a gripper, the transfer member being set opposite to a photosensitive member, where the steps of development and transfer are carried out four times, finally obtaining a full-color image.
In recent years, as recording mediums for full-color copying, it has become increasingly necessary to expand materials to various ones including not only usual paper and overhead projector (OHP) films but also cardboards and small-sized sheets of paper such as cards and postcards. In the above method making use of four photosensitive members, the recording medium is straight transported, and hence the method can be widely applied to a variety of recording mediums. However, since a plurality of toner images must be superimposed accurately at given positions on the recording medium, there is such a problem that even any slight mis-registration makes it difficult to obtain high-quality images in a good reproducibility, requiring a complicated mechanism for transporting the recording medium to make the necessity for reliability higher and the number of component parts larger. Moreover, when cardboards having a large basis weight are used in a method in which the recording medium is wound around the transfer member surface by suction, the rear end of the recording medium may cause faulty attraction because of a strong stiffness of the recording medium, consequently undesirably causing faulty images ascribable to transfer. Similar faulty images may also occur on the small-sized sheets of paper.
Accordingly, as a system that can be applied in various recording mediums and can be miniaturized, a process system making use of an intermediate transfer member is proposed. For example, full-color image forming apparatus employing a drum-shaped intermediate transfer member are already known as disclosed in U.S. Pat. No. 5,187,526 and Japanese Patent Application Laid-open No. 4-16426.
The above U.S. Pat. No. 5,187,526 discloses that a high image quality can be achieved when an intermediate transfer roller comprising a surface layer formed of polyurethane as a base material is made to have a volume resistivity below 109 xcexa9xc2x7cm and a transfer roller comprising a similar surface layer is made to have a volume resistivity of 1010 xcexa9xc2x7cm or above. In such a system, however, a high-output electric field is necessary for imparting transfer charges to the toner in a sufficient quantity when the toner is transferred to the recording medium, and hence a conductivity-providing agent is dispersed in the surface layer formed of polyurethane. This surface layer may locally cause breakdown to undesirably cause a conspicuous image disorder in halftone images where the toner is laid in a smaller quantity. Moreover, in an environment of high humidity which is higher than 60%RH (relative humidity), the application of such a high voltage tends to cause faulty transfer because transfer electric currents may leak as recording mediums are made to have a lower resistance. Meanwhile, in an environment of low humidity which is lower than 40%RH (relative humidity), it may also cause faulty transfer ascribable to non-uniform resistance of recording mediums.
In addition, in the full-color image forming apparatus in which a plurality of toner images are transferred, the toners on the intermediate transfer member are in a larger quantity than that in black-and-white copying and necessarily remain as transfer residual toners in a larger quantity. Hence, it becomes necessary to strengthen the shear force or rubbing force acting between the intermediate transfer member and a cleaning member. Accordingly, when color toners having a good fixing performance are used, the melt-adhesion or filming of toner tends to occur on the surface of the intermediate transfer member, so that transfer efficiency may become poor and problems on color uniformity and color balance tend to occur because of four color toner images not uniformly transferred in full-color copying. Thus, it has been difficult to stably form full-color images with a high image quality. That is, also in this transfer step, toners having well balanced fixing performance and running performance are desired.
As publications disclosing the relationship between the toner and the constitution employing an intermediate transfer member, named are Japanese Patent Applications Laid-open No. 59-15739 and No. 59-5046. These publications, however, only indicate that a toner with particle diameters of 10 xcexcm or smaller is transferred in a good efficiency by the use of an adherent intermediate transfer member. Usually, in the system employing the intermediate transfer member, toner visible images must be once transferred from the photosensitive member to the intermediate transfer member and further again transferred from the intermediate transfer member to the recording medium, where the transfer efficiency of toner must be made much higher than that in the above conventional processes. Especially when a full-color copying machine is used in which a plurality of toner images are transferred after development, the toners on the photosensitive member are in a larger quantity than a monochromatic black toner used in black-and-white copying machines, and it is difficult to improve the transfer efficiency only by using conventional toners. Moreover, when conventional toners are used, the melt-adhesion or filming of toners may occur on the surfaces of the photosensitive member and intermediate transfer member because of the shear force or rubbing force acting between the photosensitive member or intermediate transfer member and the cleaning member and/or between the photosensitive member and the intermediate transfer member, so that the transfer efficiency may become poor and problems on color uniformity and color balance tend to occur because of four color toner images not uniformly transferred in full-color copying. Thus, it has been difficult to stably form full-color images with a high image quality.
In addition, as toners set in usual full-color copying machines, all the color toners are required to be well color-mixed in the step of fixing. From this viewpoint, the improvement of color reproducibility and the transparency of OHP images are important, and, compared with black toners, it is commonly preferable to use in color toners sharp-melt and low-molecular weight resins. In usual black toners, as previously stated, release agents having a relatively high crystallizability as typified by polyethylene wax and polypropylene wax are used in order to improve the high-temperature anti-offset properties at the time of fixing. In the full-color toners, however, as previously stated, this crystallizability of release agents may cause a great damage in the transparency of OHP toner images when outputted. For this reason, usually, silicone oil is uniformly applied to the heat fixing roller without addition of any release agents as color toner constituents so that the high-temperature anti-offset properties can be improved. However, an excess silicone oil may adhere to the surface of the recording medium having fixed toner images thus formed, to undesirably give users disagreeable feeling when used. Thus, the full-color image formation making use of the intermediate transfer member, having many contact portions, has many difficult problems at present. The above Japanese Patent Applications Laid-open No. 59-15739 and No. 59-5046 do not present any proposal for contriving the toners or intermediate transfer member in this regard.
Meanwhile, when the toner image formed on the photosensitive member in the developing step is transferred to the recording medium in the transfer step and when the transfer residual toner remains on the photosensitive member as previously stated, it becomes necessary for the transfer residual toner to be removed by cleaning in the cleaning step and stored in a waste toner container. In this cleaning step, blade cleaning, fur brush cleaning and roller cleaning have been used as cleaning means. Such means are those by which the toner remaining after transfer (transfer residual toner) is mechanically scraped off or blocked up so that it is collected in the waste toner container. Hence, because of such a member that is brought into pressure touch with the photosensitive member, unavoidable problems have tended to arise. For example, if a cleaning member is strongly pressed, the surface of the photosensitive member is worn to shorter the lifetime of the photosensitive member.
When viewed from the aspect of apparatus, the whole apparatus must be made larger in order to provide such a cleaning means. This has been a bottleneck in attempts to make apparatus compact. In addition, from the viewpoint of ecology, a system that may produce no waste toner is long-awaited in the sense of effective utilization of toners.
As publications disclosing techniques relating to a cleanerless system, Japanese Patent Applications Laid-open No. 59-133573, named are No. 62-203182, No. 63-133179, No. 64-20587, No. 2-302772, No. 5-2289, No. 5-53482 and No. 5-61383. None of these, however, refer to any desirable toner composition.
In a cleaning-at-development system (or cleaning-cum-development) having substantially no cleaning assembly, it is essential to provide a system in which the surface of the photosensitive member is rubbed with a toner and a toner carrying member. This may cause deterioration of the toner, deterioration of the toner carrying member surface and deterioration or wear of the photosensitive member surface as a result of long-term operation, leaving the problem of deterioration of running performance. Any conventional toners attaching importance to fixing performance can not well solve such problems. Thus, it is also sought to provide a technique that can achieve both fixing performance and running performance of toners.
In respect of non-magnetic one-component contact development, Japanese Patent Application Laid-open No. 7-281485 discloses a technique of a polymerization toner having the effect of restraining the deterioration of the toner carrying member surface and the deterioration of the photosensitive member surface. However, resins used therein are those commonly available, and the publication does not mention at all any influence coming from the composition of resin. It also has no disclosure relating to the compatibility with fixing performance.
Japanese Patent Application Laid-open No. 8-305074 discloses a cleanerless image forming method making use of a toner having a specific particle shape and having 1,000 ppm or less of residual monomers. There, however, is room for further improvement in relation to the adhesion of toner to the surface of the photosensitive member or toner carrying member.
An object of the present invention is to provide a toner solving the problems arising in prior art, and an image forming method employing such a toner.
Another object of the present invention is to provide a toner for developing electrostatic images which has a high running performance and a high transfer efficiency, and an image forming method employing such a toner.
Still another object of the present invention is to provide a toner for developing electrostatic images which may less vary in charging performance depending on environment and has a high transfer efficiency, and an image forming method employing such a toner.
A further object of the present invention is to provide an image forming method that can greatly improve running performances (or durability) such as resistance to toner deterioration and resistance to melt-adhesion of toner while maintaining low-temperature fixing performance by using a special toner in a contact development type image forming process employing a cleanerless system or an intermediate transfer member.
To achieve the above objects, the present invention provides a toner comprising a binder resin, a colorant and a wax, wherein;
the binder resin has a polycarbonate resin in an amount of from 0.1% by weight to 50.0% by weight and a resin other than the polycarbonate resin in an amount of from 50.0% by weight to 99.9% by weight, based on the weight of the binder resin; and
in a molecular weight distribution as measured by gel permeation chromatography (GPC) of tetrahydrofuran(THF)-soluble matter, the toner contains in an amount of 15.0% by weight or less based on the weight of the toner a component having in its structure a repeating unit of the polycarbonate resin, contained in components having a molecular weight of 1,000 or less.
The present invention also provides an image forming method comprising the steps of;
(I) externally applying a voltage to a charging member to electrostatically charge an electrostatic latent image bearing member;
(II) forming an electrostatic latent image on the electrostatic latent image bearing member thus charged;
(III) developing the electrostatic latent image formed on the electrostatic latent image bearing member by using a toner to form a toner image;
(IV) transferring the toner image formed on the electrostatic latent image bearing member, to a recording medium via, or not via, an intermediate transfer member; and
(V) heat-fixing to the recording medium the toner image transferred to the recording medium;
the toner comprising a binder resin, a colorant and a wax, wherein;
the binder resin has a polycarbonate resin in an amount of from 0.1% by weight to 50.0% by weight and a resin other than the polycarbonate resin in an amount of from 50.0% by weight to 99.9% by weight, based on the weight of the binder resin; and
in a molecular weight distribution as measured by gel permeation chromatography (GPC) of tetrahydrofuran(THF)-soluble matter, the toner contains in an amount of 15.0% by weight or less based on the weight of the toner a component having in its structure a repeating unit of the polycarbonate resin, contained in components having a molecular weight of 1,000 or less.